1. Field of the Invention
The present invention relates to an optical information recording/reproducing apparatus and an optical information recording/reproducing method, in particular, an optical information recording/reproducing apparatus and an optical information recording/reproducing method with which a relative tilt between a recording medium and an objective lens is corrected.
2. Related Background Art
It is well known that a warpage or an inclination (tilt) of an optical disk that is a recording medium lowers information recording/reproducing performance. In view of this problem, tilt correction means shown in FIGS. 7 to 9 have conventionally been used, for instance.
FIG. 7 is an explanatory diagram of an example of a tilt detection means and a tilt correction means. An optical pickup 12 records/reproduces information onto/from an optical disk 11. The optical pickup 12 images a minute light spot on a predetermined track of the optical disk 11 using an objective lens 13, condenses reflection light thereof using the objective lens 13, guides a pencil of light onto a sensor 17 through a mirror 15 and a sensor lens 16, and performs recording/reproduction of information using a photoelectrically converted signal. A tilt actuator 14 holds the objective lens 13 and performs focusing and tracking in order to cause the light spot to trace the predetermined track. When doing so, as indicated by an arrow 11A, the optical disk 11 generates a tilt mainly in a disk radial direction due to changes in temperature, humidity, or the like, which results in a situation where the optical disk 11 does not exist perpendicular to the optical axis of the objective lens 13 of the optical pickup 12. When such a tilt occurs, a side lobe due to a coma aberration occurs to the light spot imaged on the predetermined track of the optical disk and crosstalk of information occurs between adjacent tracks, which makes it difficult to perform precise information recording/reproduction.
In order to solve this problem, in the optical pickup 12 shown in FIG. 7, a tilt sensor 18 is provided which is an element that measures a disk tilt amount of the optical disk 11. The tilt sensor measures the disk tilt amount of the optical disk 11 by emitting a pencil of light using a light emitting element 18-a and receiving and detecting reflection light thereof from a disk surface using a multi-segment light receiving element 18-b. For instance, the light receiving element 18-b is divided into two segments in a radial direction and is adjusted so that the same amount of light is incident on both of the segments when the disk tilt amount is zero (indicated by a dotted line) and the amount of light incident on one of the segments becomes larger than the amount of light incident on the other thereof when a disk tilt occurs (indicated by a solid line). With this construction, it becomes possible to measure a disk tilt amount. In addition, although not illustrated in the drawing, the tilt actuator 14 has a magnetic circuit and is given a function of tilting the objective lens 13 in the direction of an arrow 14A by devising the magnetic circuit. By giving a disk tilt amount corresponding to the tilt of the optical disk detected by the tilt sensor 18 to the objective lens 13 so that the optical disk 11 and the objective lens 13 become approximately parallel to each other, the coma aberration occurring to the light spot is reduced and precise information recording/reproduction becomes possible.
FIG. 8 is an explanatory diagram of another example of the tilt correction means.
An optical pickup shown in FIG. 8 also has conventionally been proposed in which a liquid crystal tilt correction element 19 is used instead of the tilt actuator 14 having the function of inclining the objective lens. Each constituent element that is the same as in FIG. 7 is given the same reference numeral. An actuator 14′ is an ordinary actuator that performs focusing and tracking in order to cause a light spot to trace a predetermined track. A surface, through which a pencil of light passes, of the liquid crystal tilt correction element 19 is composed of a multi-segment liquid crystal cell and it is possible to drive segments thereof independently of each other. With this construction, it becomes possible to give a desired optical phase difference to the pencil of light passing through the liquid crystal cell. Therefore, by controlling the segments in accordance with a disk tilt amount of an optical disk 11 detected by a tilt sensor 18 so that a coma aberration resulting from a tilt of the optical disk 11 is canceled out, the coma aberration occurring to the light spot is reduced and precise information recording/reproduction becomes possible. It is possible to drive the liquid crystal tilt correction element 19 with a relatively small amount of electric power, so this element 19 is suited for implementation in a portable optical disk apparatus.
The tilt sensor 18 is capable of measuring a tilt of the optical disk 11 with a simple structure and therefore is widely used across the optical disk field. With the tilt sensor 18, however, the relative tilt between the objective lens 13 and the optical disk 11 is not directly measured, so there is a problem that it is impossible to perform correction of a coma aberration with a sufficient degree of precision. Also, when the tilt sensor 18 is used in combination with a tilt correction means, such as the tilt actuator 14 and the liquid crystal tilt correction element 19, and the relative tilt itself between the optical disk 11 and the optical pickup 12 is not corrected, it is difficult to perform tilt control with a closed loop structure and it is impossible to attain a sufficient degree of correction accuracy.
In view of these problems, as an alternative to the tilt sensor 18 separately installed in the optical pickup 12, a method is proposed with which a disk tilt amount is measured with precision directly from a pencil of reflection light from an optical disk (see Japanese Patent Application Laid-Open No. 2003-045058, for instance). An example thereof is shown in FIG. 9. Each constituent element that is the same as that in FIG. 7 is given the same reference numeral. FIG. 9 is an explanatory diagram of an example of a tilt detection means and a tilt correction means and illustrates an enlarged view of a sensor 17 that receives a pencil of reflection light from an optical disk and a method of generating a tilt detection signal. The sensor 17 is composed of six sensor segments 17a to 17f and the lateral direction in this drawing corresponds to the radial direction of the optical disk. A diffraction pattern 20 from a tracking guide groove of the optical disk is superimposed on a light spot on the sensor and FIG. 9 shows a case where a light spot on the optical disk is positioned at the center of the guide groove. A push-pull system is used for tracking and, after each sensor segment output passes through each amplifier (not shown), a tracking error signal 26 is generated from a difference between an output sum of the right sensor segments and an output sum of the left sensor segments.tracking error (TE) signal=(17d+17e+17f)−(17a+17b+17c)  (1)
A differential amplifier 25 generates a differential output from the right and left sensor segment outputs and the tracking error signal 26 is obtained from the output.
Also, a tilt detection signal 24 is generated from a difference between the respective outputs of the inner left and right sensor segments (17b and 17e) and a difference between the respective output sums of the outer left and right sensor segments (17a, 17c, 17d, and 17f).tilt detection signal=(17a+17c−17d−17f)−k×(17b−17e)  (2)
where k is a constant that is greater than “1”. A differential amplifier 21 finds the difference between the respective output sums of the outer left and right sensor segments (17a, 17c, 17d, and 17f) and a differential amplifier 22 finds the difference between the respective outputs of the inner right and left sensor segments (17b and 17e). Also, a differential amplifier 23 finds a difference between an output of the differential amplifier 21 and an output of the differential amplifier 22 multiplied by “k”. When tracking control is closed, it is possible to detect the tilt detection signal 24 precisely. Consequently, in contrast to the construction using the tilt sensor 18 where tilt detection accuracy is on the order of 0.5° at most, the tilt detection accuracy is improved to 0.1°. This is because it is possible to directly observe a pencil of reflection light from the optical disk. Also, when the tilt detection signal obtained with Equation (2) given above is used in combination with a tilt correction means such as the tilt actuator 14 and the liquid crystal tilt correction element 19, it becomes possible to obtain a loop control structure with ease, which makes it possible to improve the tilt correction accuracy.
It is possible to roughly estimate a coma aberration due to an inclination of an optical disk using the following equation:coma aberration=d×NA3/λ  (3)where d is a disk substrate thickness, NA is the numerical aperture of an objective lens, and λ is the wavelength of a light source.
In recent years, in order to improve the recording density of optical disks, development of a violet semiconductor laser has remarkably advanced and is now reaching the level of practical application. In addition, many attempts have been made to increase the objective lens NA. However, when the violet semiconductor laser is used, a comma aberration occurring due to a disk tilt is increased by around 1.6 times as compared with a case where a red laser is used. Similarly, when the NA is increased from 0.6 to 0.65, the coma aberration is increased by around 1.3 times. Also, it can be understood that when both of these operations are implemented at the same time in an optical pickup for DVDs (λ=660 nm, NA=0.6), the coma aberration occurring due to a disk tilt is increased by around 2.1 times.
FIG. 10 provides estimation of such influences through simulation and illustrates how a tracking error signal level is lowered due to a relative tilt between a recording medium and an objective lens. In FIG. 10, λ is set at 405 nm, the NA is set at 0.65, and the disk substrate thickness is set at 0.6 mm. Also, a disk tilt amount is plotted on the horizontal axis and a tracking error signal amplitude of a push-pull system is plotted on the vertical axis. Further, a land-groove recording system is used in which a track pitch is set at 0.32 μm (pitch between a land center and a groove center=0.32 μm) and the depth of guide grooves is set at 30 nm. As to the tracking error signal amplitude, a result of division of a difference output of a two-segment sensor by a sum output thereof is used and normalization has been made by setting an amplitude in the case where no disk tilt exists at “1”. It can be seen from this drawing that when a disk tilt of 0.5° occurs, the tracking error signal amplitude is lowered to around one half. From this fact, it can be understood that precise tilt correction is indispensable.
The tilt detection accuracy of the tilt sensor 18 greatly depends on the installation accuracy of the sensor. Generally, however, when the tilt sensor 18 is applied to a compact and portable optical pickup, the installation position of the tilt sensor 18 is restricted and it is difficult to improve the installation accuracy thereof. Also, when the tilt sensor 18 is used in combination with a tilt correction means, such as the tilt actuator 14 and the liquid crystal tilt correction element 19, and a relative tilt itself between the optical disk and the optical pickup is not corrected, it is difficult to perform tilt control with a closed loop structure, the tilt correction accuracy obtained is on the order of 0.5° at most, and it is impossible to obtain sufficient tilt correction accuracy under the conditions described above. On the other hand, with the method with which a disk tilt amount is measured with precision directly from a pencil of reflection light from an optical disk, the tilt detection accuracy is improved to around 0.1° and when this method is combined with a tilt correction means such as the tilt actuator 14 and the liquid crystal tilt correction element 19, it becomes possible to obtain a closed loop control structure with ease, which makes it possible to improve the accuracy. With this method, however, it is required to drive a tracking servo in order to measure a disk tilt amount. When the disk tilt amount is 0.5° or more, the amplitude of a tracking error signal is lowered to around one half as shown in FIG. 10, so it becomes difficult to drive the tracking servo, a possibility that tracking pull-in will end in failure is increased, and it becomes difficult to measure the disk tilt amount.